The invention relates to a power brake system, in particular a dual-circuit power brake system, for a motor vehicle.
In power brake systems with brakes that can be activated hydraulically, the force of the driver's foot on a brake pedal acts directly via a hydraulic circuit on a plurality of brakes, if appropriate with the assistance of a brake servo. Purely mechanical activation systems such as a Bowden cable are generally used as a parking brake. It is also known to activate the parking brake electromechanically.
An additional activation force, which relieves the driver, is applied in the brake servo. However, it is necessary to make sure that the braking effect continues to be ensured even if the brake servo fails.
The electrohydraulic brake systems with which wheel-specific control of the brakes is implemented by way of electromechanically activated plungers are known from the prior art. This brings about a considerable improvement in the controllability and, therefore, the effectiveness of the brakes. In the event of a failure of the electrical system or if other system-related faults occur, the entire protection of the braking effect has to be ensured by an additional brake circuit, which usually acts on the front axle and is activated mechanically or hydraulically.
Wheel brakes, which can be activated in a purely electromechanical fashion, are also known, but such wheel brakes have hitherto not been used exclusively owing to the stringent requirements made in terms of safety of the electrical power supply and the associated technical problems and high costs. Consideration is also being given to combining hydraulically activated brakes at the front axle with electromechanically activated brakes with an integrated parking brake function at the rear axle.
Electromechanical brakes, which are used as a parking brake and into which an electromechanical parking brake is integrated, provide, in this function, certain advantages over customary, hydraulically activated activation systems and additional purely mechanically acting activation systems. These advantages are, for example, improved operator control comfort and a smaller requirement for space in the passenger compartment due to the elimination of a hand brake lever.
The systems which are known by the term “brake-by-wire” and whose characteristic is the decoupling of the activation device and the transmission device, and which generally also use brakes which can be activated electromechanically have, despite the advantages, been used only rarely until now due to the abovementioned misgivings about safety.
The invention is based on the object of providing a power brake system which can be implemented easily and cost-effectively while satisfying the necessary safety aspects.
This object is achieved by a power brake system, in particular a dual-circuit power brake system for a motor vehicle, having at least one or more brake circuits which each have at least one or more brakes that are each assigned to a wheel of the vehicle. The brakes can be activated by way of a brake pedal to which a brake servo, which is operatively connected to the brake pedal, is assigned. In each case, a separate brake servo is assigned to each brake of at least one brake circuit of the brake system, which brake servos can be activated by way of the brake pedal. Moreover, the brake servos can be actuated independently of one another with respect to their brake boosting effect.
According to the invention, a brake system, in particular a dual-circuit power brake system, is provided for a motor vehicle, having at least one or more brake circuits that each have at least one or more brakes which are each assigned to a wheel of the vehicle, wherein the brakes can be activated by a brake pedal to which a brake servo, which is operatively connected to the brake pedal, is assigned. In each case, a separate brake servo is assigned to each brake of at least one brake circuit or a plurality of brake circuits of the brake system, which brake servos can be activated together, in particular simultaneously, by way of the brake pedal, but can preferably be activated independently of one another with respect to their brake boosting effect.
In principle, the invention provides a “brake-by-wire” system which permits selected braking of any connected wheel.
Use is preferably made here of electrohydraulic brake servos, which are arranged such that the force of the driver's foot acting on the brake pedal is distributed to all the brake servos uniformly or according to a predefined rule.
Each electrohydraulic brake servo has an actuator which is provided with an electromechanical drive. When the brake pedal is activated, an actuating force is added or subtracted via the actuator for each brake in order to set the distance which is predefined by a brake control system. In addition, direct activation of the brakes by the brake system without driver intervention is possible.
In this way, even in the event of a failure of the electrical energy or of the communication connection, the mechanics/hydraulic basic function of the brakes remains unaffected. The brake system, which is modified according to the invention, is therefore a “brake-by-wire” system providing maximum fail safety.
It is generally sufficient if the two brakes of the front axle of the motor vehicle have the described mechanical through-engagement since one brake circuit therefore remains fully active. The brakes of the second brake circuit, usually the brakes assigned to the brake circuit of the rear axle, can be embodied directly as electromechanical brakes, with direct engagement on the respective wheel. In this case, the electromechanical brakes of the rear axle can be equipped with an integrated parking brake function, as a result of which maximum functionality of the brake system, associated with a simple design which is less susceptible to faults and maximum fail safety, is achieved.
The central element of an actuator used herein is a threaded spindle, preferably a ball screw spindle, on a first end of which the brake pedal which is activated by the force of the driver's foot acts, while the applied force is transmitted to a master brake cylinder which is arranged at the other end of the spindle. A spindle nut, which engages around the threaded spindle, is driven by an electric motor, as a result of which, depending on the sense of rotation, an activation force is added to the force supplied by the brake pedal (or subtracted therefrom). Given corresponding dimensioning of the electric drive, the activation pressure which is generated by the brake cylinder can be boosted to a desired extent.
The advantage of an electrohydraulic brake servo is that the activation energy no longer has to be generated by additional pumps, but rather is made available directly by electrical means. The brake pressure of individual wheels is controlled independently, for example during ABS control processes, by way of additional valves and electrically driven hydraulic pumps.
In the embodiment according to the invention, there is a separate spindle system for each wheel. The spindle system permits wheel-specific intervention directly via the electromotive spindle drive. According to the invention, at least two of the spindle actuators are activated together by way of the brake pedal, wherein the force which is applied by the brake pedal is distributed among the spindles with the same predefined ratio in every working position of the individual spindles. The electric drive can add or subtract an actuating force at the respective spindle individually and according to demand.
The invention also provides a brake system in which each brake servo has an electromechanical drive which includes a ball screw spindle, around which an electrically drivable spindle nut engages. The ball screw spindle communicates with a hydraulic cylinder by means of which the respectively assigned brake can be activated hydraulically, wherein each of the brake servos is configured such that an activation of the brakes is possible solely by means of the ball screw spindle even if the electrically drivable spindle nut fails.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.